1. Field of the Invention
The present invention relates to a ladder type piezoelectric filter to be suitably used in a filter circuit of portable and mobile radio communication equipment such as an automobile telephone set and, more particularly, the present invention relates to a ladder type piezoelectric filter to be most suitably used for digital telecommunications purposes.
2. Prior Art
Known ladder type piezoelectric filters of the type under consideration comprise a plurality of series resonators and parallel resonators arranged alternately between terminals including input terminals, output terminals and grounding terminals. In any of these filters, the resonant frequency of the series resonators and the antiresonant frequency of the parallel resonators are made to agree with each other and the difference between the resonant frequency and the antiresonant frequency is held to a constant value for both the series and parallel resonators.
FIG. 1 of the accompanying drawings is a circuit diagram of a ladder type piezoelectric filter circuit comprising six resonators, or three series resonators S having a resonant frequency of about 455 kHz and an antiresonant frequency of about 477 kHz and three parallel resonators P having a resonant frequency of about 433 kHz and an antiresonant frequency of about 455 kHz, the difference .DELTA.f between the resonant frequency and the antiresonant frequency being held to 22 kHz for both the series resonators and the parallel resonators. In FIG. 1 L1 denotes an input and output circuit and L2 a grounding circuit.
FIGS. 2 and 3 are graphs showing respectively the filtering performance and the group delay time characteristic of a ladder type piezoelectric filter having such a configuration. While the filter operates satisfactorily in terms of safeguard attenuation as shown in FIG. 2, its group delay performance lacks flatness as clearly seen from FIG. 3. To improve the group delay time characteristic has been required.
On the other hand, piezoelectric filters are required to show a short group delay time in order to minimize the phase distortion. However, 6-elements type piezoelectric filters as described above have the drawback of a long group delay time, although they show an improved attenuation performance.
While it is known that the performance of a piezoelectric filter of the type under consideration is improved in terms of group delay time by using piezoelectric resonators made of a low Qm piezoelectric material, this technique of improving the group delay time to make it show a desired value is practically not feasible because it requires the development of a new material and a subsequent series of cumbersome operations of quantifying the performance of the developed material.
Additionally, attempts have been made to improve the performance of a known piezoelectric filter of the type under consideration by inserting an elastic sheet between any adjacently disposed resonators and/or by specifically treating the terminal plates. However, such techniques of adding physical means are accompanied by the problem of an increased number of components and an increased number of processing steps.
It is therefore, an object of the present invention to provide a ladder type piezoelectric filter comprising six resonators and having a circuit configuration such that the group delay time characteristic of the filter can easily be improved.